Introduce numeric error codes in dtu webapi

include/WebApi_errors.h

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#pragma once
+
+enum WebApiError {
+    GenericBase = 1000,
+    GenericSuccess,
+    GenericNoValueFound,
+    GenericDataTooLarge,
+    GenericParseError,
+    GenericValueMissing,
+
+    DtuBase = 2000,
+    DtuSerialZero,
+    DtuPollZero,
+    DtuInvalidPowerLevel,
+};